Ductile Iron Having Cobalt

ABSTRACT

Known cast iron alloys have application limits with regard to temperature. A cast iron including cobalt is provided. Through the use of cobalt, an optimal ferritic microstructure is achieved such that with an alloy having silicon 2.0-4.5 wt %, cobalt 0.5-5 wt %, carbon 2.0-4 wt %, molybdenum 0.3-1.48 wt %, manganese ≦0.5 wt %, nickel ≦0.5 wt %, the remainder iron, wherein the proportion of silicon, cobalt, and molybdenum is preferably less than 7.5 wt %, the application limits are shifted to high temperatures.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2009/061457, filed Sep. 4, 2009 and claims the benefitthereof. The International Application claims the benefits of Germanapplication No. 10 2008 051 042.4 DE filed Oct. 9, 2008. All of theapplications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a cast iron comprising cobalt as claimed in theclaims and to a component as claimed in the claims.

BACKGROUND OF INVENTION

The known cast iron alloys now employed (so-called GJS nodular cast ironalloys) primarily use silicon and molybdenum to increase the creepstrength, scaling resistance and endurance strength. Over time, however,these elements lead to a significant decrease in the toughness.

Molybdenum furthermore exhibits a very high susceptibility tosegregation.

SUMMARY OF INVENTION

It is therefore an object of the invention to specify an alloy and acomponent, which overcome the aforementioned disadvantages and havebetter mechanical strengths over the service life.

The object is achieved by an alloy as claimed in the claims and acomponent as claimed in the claims.

The dependent claims list further advantageous measures which mayadvantageously be combined with one another in any desired way.

The invention consists in the fact that cobalt can partially replacemolybdenum. The working limitations presented by the previous GJS alloycan therefore be overcome. The alloy according to the invention has highelongations for the application field in the temperature range of 450°C.-550° C., and has the following composition (in % by weight):

silicon 2.0%-4.5%cobalt 0.5%-5%carbon 2.0%-4.5%molybdenum 0.3%-1.48%,remainder iron.

Advantageously, the proportion of silicon, cobalt and molybdenum is≦7.5% by weight, in particular ≦6.0% by weight.

Preferably, the proportion of cobalt in the alloy lies between 0.5% byweight and 1.5% by weight cobalt.

Advantageous mechanical values are achieved for the alloy respectivelywhen the cobalt content is 0.5% by weight, 1.0% by weight cobalt, 1.5%by weight cobalt and 2.0% by weight cobalt.

The addition of molybdenum to the alloy (preferably 0.5%-0.8%) has apositive influence on the high-temperature strength (Rp0.2 and Rm in theelevated temperature range) and the endurance behavior (creep strength).

Magnesium increases the castability and is preferably present in anamount of at least 0.02% by weight, at most 0.07% by weight.

Depending on the application, chromium is preferably present in anamount of at least 0.01% by weight, but at most 0.05% by weight, andthis increases the oxidation resistance.

The alloy may contain further elements.

Preferably, however, the alloy consists of iron, silicon, cobalt andcarbon.

Particular advantages are also achieved when the alloy consists of iron,silicon, cobalt, carbon and manganese.

Further advantages are obtained with an alloy which consists of iron,silicon, cobalt, carbon and optional admixtures of molybdenum, manganeseand/or nickel.

The alloy optionally contains small minimum admixtures of phosphorus0.005% by weight, sulfur 0.001% by weight, magnesium 0.01% by weight,

which have a positive influence on the castability and/or the formationof the nodular graphite, but also must not be excessively high sinceotherwise the negative influences prevail.

Furthermore, there is preferably no chromium (Cr) in the alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will be explained in more detailwith reference to the following figures:

FIG. 1 shows a cast part in a micrograph,

FIG. 2 shows a steam turbine,

FIG. 3 shows a gas turbine.

DETAILED DESCRIPTION OF INVENTION

The alloy has an almost optimal ferritic microstructure with nodulargraphite (FIG. 1).

The table shows exemplary alloys which have improved mechanicalproperties.

Cobalt 0 0.56 0.99 1.44 Carbon 3.63 3.67 3.65 3.67 Silicon 2.45 2.422.49 2.41 Manganese 0.067 0.036 0.03 0.029 Phosphorus 0.007 0.006 0.0070.007 Sulfur 0.009 0.006 0.008 0.008 Magnesium 0.044 0.04 0.05 0.049Molybdenum 0.87 0.5 0.3 0.3

Even small proportions of cobalt and molybdenum improve the mechanicalcharacteristics.

FIG. 2 shows a steam turbine 300, 303 having a turbine shaft 309extending along an axis of rotation 306.

The steam turbine comprises a high-pressure turbine part 300 and amedium-pressure turbine part 303, each with an inner housing 312 and anouter housing 315 enclosing the latter. The high-pressure turbine part300 is, for example, configured in pot design. The medium-pressureturbine part 303 is, for example, configured to be twin-streamed. It islikewise possible for the medium-pressure turbine part 303 to beconfigured to be single-streamed.

A bearing 318 is arranged along the axis of rotation 306 between thehigh-pressure turbine part 300 and the medium-pressure turbine part 303,the turbine shaft 309 comprising a bearing region 321 in the bearing318. The turbine shaft 309 is mounted on a further bearing 324 besidethe high-pressure turbine part 300. In the region of this bearing 324,the high-pressure turbine part 300 comprises a shaft seal 345. Theturbine shaft 309 is sealed relative to the outer housing 315 of themedium-pressure turbine part 303 by two further shaft seals 345. Betweena high-pressure steam intake region 348 and a steam outlet region 351,the turbine shaft 309 in the high-pressure turbine part 300 comprisesthe high-pressure rotor blading 357. With the associated rotor blades(not shown in more detail), this high-pressure rotor blading 357constitutes a first blading region 360.

The medium-pressure turbine part 303 comprises a central steam intakeregion 333. Associated with the steam intake region 333, the turbineshaft 309 comprises a radially symmetric shaft shield 363, a coverplate, on the one hand to divide the steam flow into the two streams ofthe medium-pressure turbine part 303 and also to prevent direct contactof the hot steam with the turbine shaft 309. In the medium-pressureturbine part 303, the turbine shaft 309 comprises a second bladingregion 366 with the medium-pressure rotor blades 354. The hot steamflowing through the second blading region 366 flows from themedium-pressure turbine part 303 out of a discharge port 369 to alow-pressure turbine part (not shown) connected downstream.

The turbine shaft 309 is composed for example of two turbine shaft parts309 a and 309 b, which are connected firmly to one another in the regionof the bearing 318. Each turbine shaft part 309 a, 309 b comprises acooling line 372 formed as a central bore 372 a along the axis ofrotation 306. The cooling line 372 is connected to the steam outletregion 351 via a feed line 375 comprising a radial bore 375 a. In themedium-pressure turbine part 303, the coolant line 372 is connected to acavity (not shown in more detail) below the shaft shield. The feed lines375 are configured as a radial bore 375 a, so that “cold” steam from thehigh-pressure turbine part 300 can flow into the central bore 372 a. Viathe discharge line 372 also formed in particular as a radially directedbore 375 a, the steam passes through the bearing region 321 into themedium-pressure turbine part 303 and there onto the lateral surface 330of the turbine shaft 309 in the steam intake region 333. The steamflowing through the cooling line is at a much lower temperature than thetemporarily superheated steam flowing into the steam intake region 333,so as to ensure effective cooling of the first rotor blade row 342 ofthe medium-pressure turbine part 303 and the lateral surface 330 in theregion of this rotor blade row 342.

FIG. 3 shows, by way of example, a partial longitudinal section througha gas turbine 100.

In the interior, the gas turbine 100 has a rotor 103 with a shaft 101which is mounted such that it can rotate about an axis of rotation 102and is also referred to as the turbine rotor.

An intake housing 104, a compressor 105, a, for example, toroidalcombustion chamber 110, in particular an annular combustion chamber,with a plurality of coaxially arranged burners 107, a turbine 108 andthe exhaust-gas housing 109 follow one another along the rotor 103.

The annular combustion chamber 110 is in communication with a, forexample, annular hot-gas passage 111, where, by way of example, foursuccessive turbine stages 112 form the turbine 108.

Each turbine stage 112 is formed, for example, from two blade or vanerings. As seen in the direction of flow of a working medium 113, in thehot-gas passage 111 a row of guide vanes 115 is followed by a row 125formed from rotor blades 120.

The guide vanes 130 are secured to an inner housing 138 of a stator 143,whereas the rotor blades 120 of a row 125 are fitted to the rotor 103for example by means of a turbine disk 133.

A generator (not shown) is coupled to the rotor 103.

While the gas turbine 100 is operating, the compressor 105 sucks in air135 through the intake housing 104 and compresses it. The compressed airprovided at the turbine-side end of the compressor 105 is passed to theburners 107, where it is mixed with a fuel. The mix is then burnt in thecombustion chamber 110, forming the working medium 113. From there, theworking medium 113 flows along the hot-gas passage 111 past the guidevanes 130 and the rotor blades 120. The working medium 113 is expandedat the rotor blades 120, transferring its momentum, so that the rotorblades 120 drive the rotor 103 and the latter in turn drives thegenerator coupled to it.

While the gas turbine 100 is operating, the components which are exposedto the hot working medium 113 are subject to thermal stresses. The guidevanes 130 and rotor blades 120 of the first turbine stage 112, as seenin the direction of flow of the working medium 113, together with theheat shield elements which line the annular combustion chamber 110, aresubject to the highest thermal stresses.

To be able to withstand the temperatures which prevail there, they maybe cooled by means of a coolant.

Substrates of the components may likewise have a directional structure,i.e. they are in single-crystal form (SX structure) or have onlylongitudinally oriented grains (DS structure).

By way of example, iron-based, nickel-based or cobalt-based superalloysare used as material for the components, in particular for the turbineblade or vane 120, 130 and components of the combustion chamber 110.

Superalloys of this type are known, for example, from EP 1 204 776 B1,EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949.

The blades or vanes 120, 130 may likewise have coatings protectingagainst corrosion (MCrAlX; M is at least one element selected from thegroup consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an activeelement and stands for yttrium (Y) and/or silicon, scandium (Sc) and/orat least one rare earth element, or hafnium). Alloys of this type areknown from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306454 A1.

It is also possible for a thermal barrier coating to be present on theMCrAlX, consisting for example of ZrO₂, Y₂O₃—ZrO₂, i.e. unstabilized,partially stabilized or fully stabilized by yttrium oxide and/or calciumoxide and/or magnesium oxide.

Columnar grains are produced in the thermal barrier coating by suitablecoating processes, such as for example electron beam physical vapordeposition (EB-PVD).

The guide vane 130 has a guide vane root (not shown here), which facesthe inner housing 138 of the turbine 108, and a guide vane head which isat the opposite end from the guide vane root. The guide vane head facesthe rotor 103 and is fixed to a securing ring 140 of the stator 143.

1-34. (canceled)
 35. An alloy comprising (in % by weight): silicon2.0%-4.5%; cobalt 0.5%-5.0%; carbon 2.0%-4.5%; and molybdenum 0.5%-0.8%.36. The alloy as claimed in claim 35, further comprising (in % byweight): manganese ≦0.5%, nickel ≦0.5%, magnesium ≦0.09%, phosphorus≦0.07%, sulfur ≦0.04%, chromium ≦0.1%, and remainder iron.
 37. The alloyas claimed in claim 35, wherein a proportion of silicon, cobalt andmolybdenum is less than 7.5% by weight.
 38. The alloy as claimed inclaim 35, comprising 0.5% by weight to 1.5% by weight cobalt.
 39. Thealloy as claimed in claim 36, wherein the alloy comprises at least0.005% by weight manganese.
 40. The alloy as claimed in claim 36,wherein the alloy comprises a manganese content ≦0.07% by weight. 41.The alloy as claimed in claim 36, wherein the alloy comprises nomanganese.
 42. The alloy as claimed in claim 36, wherein the alloycomprises at least 0.01% by weight nickel.
 43. The alloy as claimed inclaim 36, wherein the alloy comprises no nickel.
 44. The alloy asclaimed in claim 35, wherein the alloy comprises 2.8% by weight—3.2% byweight silicon.
 45. The alloy as claimed in claim 36, wherein the alloycomprises at most 0.007% by weight phosphorus.
 46. The alloy as claimedin claim 36, which comprises at least 0.005% by weight phosphorus. 47.The alloy as claimed in claim 36, wherein the alloy comprises at most0.008% by weight sulfur.
 48. The alloy as claimed in claim 36, whereinthe alloy comprises at least 0.001% by weight sulfur.
 49. The alloy asclaimed in claim 36, wherein the alloy comprises at most 0.05% by weightmagnesium.
 50. The alloy as claimed in claim 36, wherein the alloycomprises at least 0.01% by weight magnesium.
 51. The alloy as claimedin claim 36, wherein the alloy comprises no chromium.
 52. The alloy asclaimed in claim 36, wherein the alloy comprises at most 0.03% by weightchromium.
 53. The alloy as claimed in claim 36, wherein the alloycomprises at least 0.01% by weight chromium.
 54. The alloy as claimed inclaim 35, wherein the alloy consists of iron, silicon, cobalt,molybdenum and carbon.